1. Field of the Invention
The present invention relates to the field of drilling and machining apparatus and methods, and in particular relates to the method and apparatus for numerically controlled drilling of printed circuit boards.
2. Description of the Prior Art
High speed, precision drilling has long been practiced in regard to the fabrication of printed circuit boards. Often the drilling of the required holes in the printed circuit board is the most time consuming and costly step in the fabrication process and limits the maximum output capacity of most fabricators. Industry standards have been assumed for the fabrication of printed circuit boards. Such long held industry wide beliefs have assumed that feed rates of drills are limited by the following standard.
Firstly, the practical chip load limit is 0.002 per inch revolution. Secondly, a per revolution feed rate which exceeds the thickness of the printed circuit board's copper layers will result in punching rather than a clean cutting action thereby destroying precision and jeopardizing the electrical integrity of the conductive overlays on the nonconductive board. Thirdly, higher feed rates will inevitablly create excessive burring at the top surface of the holes and will roughen hole walls. Fourthly, high speed drilling using high spindle speeds generates large amounts of heat in the printed circuit board thus causing epoxy smear. The underlying epoxy resin of which the nonconductive portion of the board is constituted, melts and is smeared across the copper layers typically disposed on the upper surface of the board. Such epoxy smear is likely to cause later plating defects.
What is needed then is a method for high speed, high precision drilling and an apparatus for performing the same which is not limited by the maximum performance parameters assumed by the industry, which does not produce epoxy smear, and which produces cleanly cut, high precision holes in printed circuit boards at a high repetition rate.